Electronic components and chips are commonly surface mounted to substrates, such as printed circuit boards, through the use of a solder interconnect or joint. In today's vehicles, this electronic packaging is commonly found in the powertrain, chassis control, and entertainment electronic modules.
Such electronic components may be of the leaded or no-lead (leadless) variety. Examples of the no-lead variety include the Leadless Chip Resistor (LCR), the Leadless Chip Capacitor (LCC), and the Leadless Ceramic Chip Carrier (LCCC). These leadless components are commonly formed of ceramic, such as Al.sub.2 O.sub.3, and mounted to a printed circuit board by a solder interconnect. The solder interconnect is generally a solder material formed of 37Pb/63Sn weight percent solder.
In contrast to solder interconnects for leaded components having compliant leads, the mismatch of coefficients of thermal expansion between a leadless component and a printed circuit board is borne by the solder interconnect, resulting in an interconnect having a higher stress/strain range and significant plastic deformation, causing the initiation and propagation of creep/fatigue coupled crack growth. As such, solder interconnects for leadless components in electronic packaging often exhibit low cycle thermal and high cycle vibrational fatigue failure under harsh under-hood environments.
The magnitude of the plastic and creep deformation is directly associated with the standoff (height) of the solder interconnect, i.e. the gap between the component and the circuit board. The larger the standoff, the less deformation that occurs thus leading to longer component and assembly lives.
In the past, glass or other types of balls have been utilized in joints between component and substrate. Such arrangements are disclosed in U.S. Pat. No. 5,147,084 issued to Behun et al. and U.S. Pat. No. 5,093,986 issued to Mandai et al. The former discloses a ball grid array pattern which uses a solder ball as a joint between the substrates in conjunction with solder paste for attaching the solder ball to each substrate, while the latter discloses forming electrodes using metal balls and conductive adhesives between two substrates. However, the processes disclosed do not control the standoff height and improve design life of the joint.
Consequently, a need has developed for a method of joining a component to a substrate which controls the standoff of the solder interconnect between the component and the substrate, which results in electronic packaging having improved reliability, cycle life and strength. These improvements are expected to translate into reduced warranty costs.